There is a patch (known just as the Lethe patch) available for Nethack that adds the river Lethe and other netherworldly interesting things to the game. The patch was written by Michael Clarke.

The river (complete with ?SoA effects! =) starts from under Medusa's level, and leads to the gates of Gehennom. This addition isn't the only one - Gehennom has got some pretty neat additions and changes, and reportedly it's much more interesting part after that. The special demon levels are bigger. The Castle, formerly the gateway to Gehennom, is now just another level in the "Lethe Gorge".

The patch adds new scrolls, too, bunch of new monsters (drawn from Cthulhu Mythos), and blunt silver weapons (for those who have religious problems using bladed weapons but would pretty much like to deal with demons properly, too).

It is
said that river was created by Hades as a gift to his citizens, since drinking from it swept away all of their memories of life on earth.

Not only would they forget their wants, troubles and pain, but they would also forget their loved ones and the joys they felt on earth in order to spare them the pain of missing them. Some also believe that the shades who were destined to return to earth in different forms, via reincarnation were instructed to drink a certain amount of the water so they could return as a 'clean slates', free of knowledge or pre-judgement.